Fiber optic, coaxial, and copper pair cables are widely used in the telecommunications industry. High pair count copper cables, containing individual twisted pairs of conductors, are frequently used as feeder cables between telephone customers and the telephone company central office. Broadband coaxial cables are often found in cable television (CATV) systems. Modern fiber optic cables, in particular, have revolutionized the long distance telecommunications industry in the United States and many other countries. Fiber optic cables are also penetrating into local telephone markets and CATV markets, displacing these older technologies.
Fiber optic cables offer numerous advantages over prior technology. For example, a fiber optic cable may provide an unrepeatered distance of 50 miles or more with currently available electronics. Fiber optic cables may transport digital light pulses for essentially noise-free communications transmission of vast quantities of information. The fibers, when used to transmit information in an analog signal form, offer wide signal bandwidths. Fiber optic cables have immunity to crosstalk and electromagnetic interference because of the dielectric composition of the individual fibers, and the cable itself may be made relatively light in weight and small in diameter, thereby substantially reducing installation costs.
One disadvantage of a fiber optic cable is that it often requires precise alignment of the individual fibers for joining cable segments by "fusion" splicing or mechanical connectors. Since the light carrying core of an individual fiber may typically be as small as 8 microns, precise tolerances must be observed when positioning the fiber for splicing. In addition, each splice is routinely acceptance tested to assure a proper splice. These splices, if not correctly performed, may cause unacceptable losses in the overall fiber optic system. Therefore, a field communication link between separated service technicians is typically desired to assist in alignment of the fibers during splicing operations and to verify proper splices.
Because of the large traffic carrying capacity of fiber optic cables, prompt service restoration by the splicing of damaged cable sections is often an economic necessity. Frequently, field service technicians need to communicate from a remote cable location to another remote location or to an equipment termination point or system repeater site. Unfortunately, the long unrepeatered distances available with fiber optic cable further complicates the problem of establishing field communications links. Access to the public switched telephone network is often not available at a remote site. Mobile radios may be used for field communication; however, radio frequencies are limited and radio equipment may be expensive and unreliable.
To assist field service technicians, the art has developed a method of providing a communications link between the technicians along the cable route by placing a copper "talk pair" in the fiber optic cable. A cable which has a core incorporating several buffer tubes, each containing one or more optical fibers, may have the talk pair placed in a spare buffer tube. An alternative approach is to place the talk pair directly in the extruded plastic jacket, as taught in U.S. Pat. No. 4,844,575 to Kinard et al. Both of these cable designs require that the inner fiber optic core of the cable be exposed to access the talk pair. To avoid severing the core, great care must be exercised when attempting to access the talk pair. For service restoration where only a few of the many individual fibers in the cable are damaged, an accidental or intentional severing of the cable core will disrupt working fibers; therefore, an attempt to access the talk pair may be undesirable.
A further disadvantage for optical fibers is that they require protection from external stretching, bending, and crushing forces. A failure to adequately protect the individual fibers may result in initial optical losses exceeding a planned system loss budget. Splices may then have to be remade or a costly electronic repeater site may need to be added to the system. Inadequate fiber protection may also cause premature failure of the fibers during their service life.
Optical fiber protection is typically provided by a cable structure which isolates the individual fibers from these potentially damaging external forces. For example, to protect the fibers from stress caused by an applied tension force, a longitudinally extending strength member such as a high tensile strength aramid yarn is frequently incorporated into the cable. To protect the cable from bending, a central rigidity member may be provided in the cable core or a rigid single tube in the core may be provided. Crush, impact, and cut-through resistance are provided by carrying the fibers in a central core and surrounding the core in a protective jacket. To increase the protection provided by the jacket, its thickness may be increased or multiple layers of jacketing may be provided. These modifications to the cable jacket increase the initial cost and weight of the cable while reducing its flexibility. Larger and less flexible cables typically increase labor and handling costs for installation as well.
An enhanced crush, impact, and cut-through resistant cable jacket for aerial coaxial cable is disclosed in U.S. Pat. No. 4,731,505 to Crenshaw et al., the teachings of which are hereby incorporated herein by reference. The jacket consists of a plurality of radially spaced longitudinal cavities each having a non-symmetrical cross-sectional geometry such that a radially applied force will be dissipated rather than transmitted to the cable core.